Many drugs that possess activities that could be useful for therapeutic and/or diagnostic purposes have limited value because they are rapidly eliminated from the body when administered. For example, many polypeptides that have therapeutically useful activities are rapidly cleared from the circulation via the kidney. Accordingly, a large dose must be administered in order to achieve a desired therapeutic effect. A need exists for improved therapeutic and diagnostic agents that have improved pharmacokinetic properties. Polypeptides that bind serum albumin are known in the art. (See, e.g., EP 0486525 B1 (Cemu Bioteknik AB); U.S. Pat. No. 6,267,964 B1 (Nygren et al.); WO 04/001064 A2 (Dyax, Corp.); WO 02/076489 A1 (Dyax, Corp.); WO 01/45746 (Genentech, Inc.).)